Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 28th January 2014, 06:35 PM   #1
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Default Inductance loading tube stage questions

I'm about to build a phono stage with LCR RIAA equalization according to Thorsten Loesch schematic. The second (output) stage is D3a, triode strapped, with 8.2K resistor at the anode, the output is fed by a capacitor(s). That part of the schematic is attached.

As a pre-amp I'm going to use an inductive passive pre-amp with Tribute IVC (Inductive Volume Control), which is an autoformer (auto-transformer), with taps to control the volume. A symbolic schematic of the IVC is also attached.

The Tribute IVC is ordered and paid for and so are most of the parts for the phono stage. Hence, there isn't going to be any change in those (the said phono stage and the IVC). I'm left with no extra money to change either of these (the said phono stage and the IVC). What I didn't order yet is the capacitor(s) for the output of the phono stage, and here I have room to make changes. Other than the value of the output capacitor(s), I'm not going to make any changes to the schematic of the phono stage. Eventually I may not like the sound of either the phono stage, or the IVC, or both. However I must make sure that (from the engineering viewpoint), the said phono stage will work properly with the said IVC.

Apart from that I have a pair of Lundahl LL 1676 line level transformers which I can use, if necessary.

In order to have not too many attached schematics in one post, I'll continue in the next post.
Attached Images
File Type: jpg 1.jpg (21.8 KB, 267 views)
File Type: jpg 2.jpg (56.9 KB, 261 views)
  Reply With Quote
Old 28th January 2014, 06:37 PM   #2
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
The D3a anode dynamic resistance in triode mode is 1.9K, so with 8.2K anode resistor, the calculated Zo (output impedance) of that stage is about 1.543K. There is a discrepancy between the calculated Zo and the Zo simulated by Tube Cad, so for the simulations of the output I used Zo of 1.52K.

The output of the phono stage is terminated by 1M resistor. Some termination is needed, for cases when the phono stage works, but it is unloaded by the pre-amp, which may be connected to a different input. 1M seems to me adequate, in the light of the conditions which will be detailed below.

The inductance of the IVC autoformer is 150H, with DCR of about 35 Ohm.

My power-amp is Pass Labs XA30.5, which has input impedance of 15K (mainly resistive) on its' Single Ended input, which is the input I use.

The output capacitor(s) of the D3a stage and the autoformer have a resonance frequency. The resonance frequency is set by the inductance, which is given, and the capacitance, which I'm free to choose. However the load of the power-amp is needed to be taken into account. But since changing the volume means choosing different taps of the autoformer, the load presented by the power-amp is changing. At full volume, there is 15K in parallel with the IVC coil. When lower taps of the autoformer are used at lower volumes, the load presented by the power-amp is a reflected one, according to the square-root of the turns ratio. (Concerning the reflected load by turns ratio, an autoformer behaves like a regular transformer). I estimate the lowest practical setting of the volume control in actual listening to be about 1/4 of full volume. Full volume may be seldom used, if at all.

So, at full volume, the IVC coil is shunted by a 15K resistor, while at 1/4 volume it shunted by a 135K (1/4 means turns ratio of 3, 3 squared is 9). For convenience in simulation, I simulated the two expected extreme cases, 15K and 150K shunting the IVC.

The first simulation is with a single output capacitor. With 0.47u capacitor and 15K shunt, there is an insignificant rise in the frequency response between about 250Hz and 30Hz, while there is an attenuation of about -2dB at 20Hz. This isn't bad, at least, it's acceptable.

However, on the second simulation with 150K shunt, there is a peak of about +13dB at about 20Hz. This peak I the resonance. It is unacceptable, so I need to move onwards.

See attached photos.
Attached Images
File Type: jpg 3.jpg (125.1 KB, 257 views)
File Type: jpg 4.jpg (181.9 KB, 253 views)
  Reply With Quote
Old 28th January 2014, 06:38 PM   #3
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
The asked for solution is a snubber. I tried various combinations of capacitors and resistor, in order to get at the best possible compromise in frequency response between the expected two extreme shunt values. I got at a snubber of 6.6u capacitor with a series resistor of 4.7K, shunted by a 1u capacitor. The schematic is in the attached photos.

With 15K shunt, there is an overall attenuation of about -0.8dB, which I don't mind at all (especially when this attenuation is only at full volume setting). Between about 550Hz and 100Hz there is a further attenuation (relative to the high audio frequencies) of about -0.2dB, which seems to me as insignificant. At 20Hz, relative to the high frequencies, there is an attenuation of about -1dB. It seems to me definitely acceptable.

With 150K shunt there is an overall attenuation of about -0.1dB, which seems to me as insignificant. However, at about 9Hz, relative to the high frequencies, there is a peak of about +0.95dB. The point is that 9Hz is somewhere in the region of the Tone Arm's resonance frequency. There may be a comfort, since between the 600R LCR RIAA equalization network and the control grid of the D3a, at 9Hz there is an attenuation of about -1dB. (At 20Hz the attenuation is about -0.3dB).

Thus, in the case of 150K shunt, at 20Hz there is an overall gain is about -0.3dB, at 9Hz the frequency response is practically flat, and from about 8Hz and below, the frequency response goes down, which is desirable.

Still I'm concerned about that peak at 9Hz, in spite the overall frequency response.
I'm not sure if it's acceptable.

All comments and suggestions are welcomed.
Attached Images
File Type: jpg 5.jpg (133.0 KB, 251 views)
File Type: jpg 6.jpg (124.9 KB, 73 views)
  Reply With Quote
Old 29th January 2014, 03:10 PM   #4
diyAudio Member
 
Join Date: Nov 2010
Why did you buy such ivc if you could know these problems?
  Reply With Quote
Old 29th January 2014, 03:58 PM   #5
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Quote:
Originally Posted by esltransformer View Post
Why did you buy such ivc if you could know these problems?
Hi,
Thank you for your comment.
I prefer technical comments and discussions in this particular thread.

Discussions concerning purchasing considerations and other non-technical topics would better be held on "The Lounge" forum.
  Reply With Quote
Old 29th January 2014, 06:40 PM   #6
diyAudio Member
 
Frank Berry's Avatar
 
Join Date: Sep 2003
Location: Midland, Michigan
Feeding an inductor with a series capacitor creates a tuned circuit.
Period.
__________________
Frank
  Reply With Quote
Old 29th January 2014, 07:05 PM   #7
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Quote:
Originally Posted by Frank Berry View Post
Feeding an inductor with a series capacitor creates a tuned circuit.
Period.
Hi,
Thank you.
Yes, it does and I know it. It says so both in my verbal description and in my simulations.
  Reply With Quote
Old 29th January 2014, 07:19 PM   #8
diyAudio Member
 
yagoolar's Avatar
 
Join Date: Oct 2004
Location: Mazowieckie, Jozefoslaw
Send a message via ICQ to yagoolar Send a message via Skype™ to yagoolar
Perhaps you could add a cathode follower between IVC and an amp. Thus you "isolate" low impedance input of the amp from phono pre. Just my 0,02 PLN.
  Reply With Quote
Old 29th January 2014, 07:34 PM   #9
diyAudio Member
 
Join Date: Nov 2010
I guess he goes from one problem to the other problem.

Just remove the ict and the sun shines again.
  Reply With Quote
Old 29th January 2014, 07:36 PM   #10
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Quote:
Originally Posted by yagoolar View Post
Perhaps you could add a cathode follower between IVC and an amp. Thus you "isolate" low impedance input of the amp from phono pre. Just my 0,02 PLN.
Hi,
Thank you.
The whole idea of having transformer-based passive pre-amp, like that IVC, is to avoid any active stage. Thus, any follower is opted out by me. As I wrote in the first post, other than the value of the output capacitors and the snubber, I'm not going to make any changes to the original schematic. It was designed by someone with deep knowledge and vast experience, which I don't have. I'm certainly not going to add any follower to the original schematic.

Anyhow, since the variations in the frequency response at 20Hz and below are about +/-1dB, and only at the extreme settings of the volume control, there is probably not much to worry about. At first I was concerned, but reviewing the situation with the frequency response, it's probably okay. It seems that the designer of that phono stage is also not worried much about the said frequency response variations.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
questions on Svetlana/Tube Town EF86 mic stage multi-volti Instruments and Amps 9 6th March 2014 09:25 PM
Tube VAS stage questions??? Junm Tubes / Valves 19 15th May 2012 05:03 AM
Plate loading questions db! Tubes / Valves 19 19th January 2011 04:27 PM
Aperiodic loading questions Josephjcole Full Range 1 15th November 2005 09:27 PM
Choke loading BJT output stage mashaffer Solid State 2 9th July 2004 02:22 PM


New To Site? Need Help?

All times are GMT. The time now is 02:58 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2